Simply supported beam question
Simply supported beam question
(OP)
I have a very simple question:
I have a series of 28' long floor joists that have bearing at each end and an LVL beam for bearing at the midpoint (14'). Typically I would specify a 'flush' LVL beam; hence the joists would be cut at 14' and hung with simpson hangers off the LVL. In determining my loads on the LVL beam I assume each 14' floor joist is a simply supported beam and each end of the floor joist carries 1/2 the load (assuming uniform loading).
Now, lets say I decide to drop the LVL beam, not cut the floor joists, and just rest the floor joists on top of the LVL. Now, I have an indeterminate beam situation with my floor joists. My Statics book says for this uniformly loaded indeterminate beam that my middle reaction is 1.25*w*L where w is my load in lbs/ft and L is my length. Thus, the middle reaction is greater and upsizes my LVL. I am trying to make sure I have this correct and intuitively I am not sure that I understand why the indeterminate situation has more loading at the middle reaction.
Thanks so much,
I have a series of 28' long floor joists that have bearing at each end and an LVL beam for bearing at the midpoint (14'). Typically I would specify a 'flush' LVL beam; hence the joists would be cut at 14' and hung with simpson hangers off the LVL. In determining my loads on the LVL beam I assume each 14' floor joist is a simply supported beam and each end of the floor joist carries 1/2 the load (assuming uniform loading).
Now, lets say I decide to drop the LVL beam, not cut the floor joists, and just rest the floor joists on top of the LVL. Now, I have an indeterminate beam situation with my floor joists. My Statics book says for this uniformly loaded indeterminate beam that my middle reaction is 1.25*w*L where w is my load in lbs/ft and L is my length. Thus, the middle reaction is greater and upsizes my LVL. I am trying to make sure I have this correct and intuitively I am not sure that I understand why the indeterminate situation has more loading at the middle reaction.
Thanks so much,






RE: Simply supported beam question
Considering just the fixed/pinned half beam, which end will see more load in a uniform load situation? Intuition (and calculation) tell you it's the fixed end. This in turn means that the center of your full beam would have more load.
Doing this will increase the size of your LVL beam, but the moments in the joist go down, so you can downsize the joist. The question is which is more economical considering the expense of the different connections as well.
If you "heard" it on the internet, it's guilty until proven innocent.
RE: Simply supported beam question
As far as the reactions on the LVL go (assuming the joists are continuous over it), I look at this a little differently than most. That reaction given above is assuming a rigid support that will NOT deflect. We know that your LVL will deflect. This means that you are being conservative by using this reaction for the LVL-and I would do that. However, when looking at the end reactions that are NOT continuous over a support, you should account for the fact the reaction at the LVL will be lower than given above and, as a result, the non-continuous end reactions will be higher. This will really only matter for required bearing distance, but I would check it.
RE: Simply supported beam question
I agree with using the simply-supported reactions for the joist ends, as long as it doesn't kill your framing plans, which I doubt it will do.
RE: Simply supported beam question
RE: Simply supported beam question
RE: Simply supported beam question
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RE: Simply supported beam question
Dik
RE: Simply supported beam question
If the LVL were flimsy, the end shear would be .5wL, again L=28ft. The end shear lies somewhere in between those two values. Unless one performs a stiffness analysis, one must use the conservative solution, or else guess.
RE: Simply supported beam question
Don Phillips
http://worthingtonengineering.com
RE: Simply supported beam question
Leave the joists continuous if you can, it will help the stiffness of the floor.
Swearingen,
Assuming fully loaded, the continuous joist moment over the support will be equal to the simple span midpoint moment.
Ken